The present invention relates to a method of modulating a laser beam.
Various types of high power lasers are at present known. However, among the various types in existence, only molecular CO.sub.2 lasers have a sufficient strength and output to enable them to be used industrially. These lasers, however, whose power may extend to tens of kilowatts, are designed for continuous operation. They may not be used, therefore, when a modulated high power beam is required, as, for example, in the case of surface treatment of metal components or pulse cutting.
Moreover, the modulation of high power beams has serious drawbacks from the thermal and mechanical point of view. Various ways of modulating a laser beam are in fact known.
One way consists in varying in an electrical manner the electrical excitation current of the emitter tube. However, the response time is too long and it is not possible to modulate the beam above 1000 variations per second using this means.
Optical modulators are also known, such as vibrating mirrors and electro-optical or opto-acoustic modulators. However, these cannot always be used, because of serious heat dissipation problems as soon as the power of the beam is greater than 100 W. Vibrating mirrors, for example, must be water cooled, which requires a high mass and limits the frequency of vibration of the mirror and, consequently, the frequency of modulation of the laser beams.
A further known procedure consists in using a disc rotating in the beam and drilled with holes or provided with slots on its periphery which enable the beam to pass through in a partial or complete manner, the beam thus being chopped up with a frequency depending on the speed of rotation of the disc. Using this known type of apertured disc, a considerable portion of the energy of the laser beam cannot be used as it is either reflected or absorbed by the opaque portions of the disc separating the successive apertures. This system has a further serious drawback in the case in which it is necessary to carry out very high frequency modulation of high power beams.
By way of example, for a beam having a diameter of 20 mm and corresponding to a power of 2 kW which is to be modulated at a rate of 10,000 pulses per second by means of a disc provided with slots having a width of 20 mm separated by opaque portions of 20 mm, the peripheral speed of the disc must be considerably greater than the speed of sound in air.